Prior to kernel 2.6.28, process timer implementation is poor. When the process timeris near to expire, threads will sum the time from all threads of the thread groupover and over again. If there are thousands of threads, kernel might hang eventually.I ran into it a couple of months ago when I tried to enable specweb2005 (Apache+PHPthread mode).

So the per-cpu itimer could improve this situation when thread number is far bigger thancpu number. I didn't retry specweb2005 with 2.6.28.